This invention relates to fire bricks used in rotary furnaces such as rotary cement kilns, and in particular relates to the form and construction thereof.
By providing concavities in a maximum surface area portion of a fire brick and striving for the early application and stabilization of the application of a coating, the fire bricks are protected, and the heat insulation effect is raised by the coating, whereby the durabillity of the fire bricks during use is improved and heat dispersion is reduced, or, in other words, a contribution is made to energy saving.
In rotary furnaces, chiefly rotary cement kilns, a device which uses a lot of energy, in view of the recent oil situation, particular attention is being paid to countermeasures to achieve energy savings.
Cement calcination furnaces have changed from vertical kilns to rotary furnaces, and the development of the technology thereof has been unprecedently spectacular, moving through the wet type, the semi-dry type and the dry type to the new dry type with preheaters, and recently to the NSP type provided with a preheater and an auxiliary furnace. As is generally known, by this progress the amount of fuel for one ton of cement clinker has been substantially reduced from 120 l/t to 80 l/t or less. However, even though such a reduction in fuel costs accompanying the development of equipment technology has been achieved, the recent energy situation makes further steps for energy saving very desirable. However the development of equipment technology is regarded as having reached its peak with the completion of the NSP system, and there is little room for improvement beyond it.
Heat input and output in a current NSP type kiln are as shown in the following table.
TABLE 1 ______________________________________ Kiln total system heat accounting, results (According to the type of kiln, average values)* Units: 10.sup.3 Kcal/Clinker ton ( ): Proportion (%) Kiln type SP Kiln NSP Kiln ______________________________________ Input Heat from burning 814.8 (97.3) 790.1 (97.2) heavy oil Others 23.0 (2.7) 22.6 (2.8) Input Heat Total 837.8 (100.0) 812.7 (100.0) Output Heat used in clinker 417.5 (49.8) 423.5 (52.1) calcination Preheater exhaust 175.8 (21.0) 165.4 (20.4) gas heat Heat taken off with 21.6 (2.6) 19.9 (2.4) the clinker Cooler exhaust 111.5 (13.3) 129.5 (15.9) gas heat Heat losses to 98.0 (11.7) 63.8 (7.9) radiation, etc. Others 13.3 (1.6) 10.5 (1.3) Output Heat Total 837.8 (100.0) 812.6 (100.0) ______________________________________ *From Report T12 of the Fuel Specialist Committee of the Cement Association, P.78 "A Survey with regard to SP Kilns" January, 1976.
Table 1 illustrates how important it is to reduce the amount of radiated heat.
This quantity of radiated heat consists mainly of that from the rotary kiln itself. That is to say, in order to save energy with these rotary furnaces some heat insulation countermeasures to reduce heat losses from the iron cladding of the rotary kiln are indispensable.
In rotary cement kilns, etc., there are cases of heat resistant fire bricks being used, but these are all chiefly in the low temperature range, and from the point of view of durability during use, such bricks are inferior to conventional fire bricks. Other than this, there is the two layer cladding method of disposing fire bricks with a low coefficient of heat transmission over the iron cladding, but "misalignment" with the inner bricks during use produces loosening and induces falling out, so this has seldom been used with large diameter kilns. Also, in the high temperature calcination zone, there have been used the so-called clog-bricks in which concavities are provided in the back of the bricks and in which a refractory heat barrier material is applied thereto, and two layer bricks in which the iron cladding of the fire bricks is in a material with a low heat transmission coefficient. However, they all lack strength and so are not suitable for long term operation.
Cement clinker adheres to the refractory material used in the calcination zone of a rotary cement kiln, and a coating is formed, and since the heat transmission coefficient of this coating is substantially lower than that of the fire bricks themselves a heat insulation effect is obtained, and the temperature of the fire bricks is lowered. Accordingly, the coating can be said to serve two purposes, namely saving energy and protecting the bricks.
The relationship between the coating thickness and iron cladding surface temperature in a rotary cement kiln is as follows.
TABLE 2 ______________________________________ Relationship between Iron Cladding Surface Temperature and Coating Thickness* Preconditions: Bricks: Quality: High Temperature Calcinated Basic Bricks, Thickness 200 mm Iron Cladding: Thickness 40 mm, Inside Diameter 5,000 mm, Blackness of the iron cladding = 0.85 Coating Inner Wall Surface Temperature: 1450.degree. C. External Air Temperature: 20.degree. C. (No Wind) Heat Transmission Coefficient (Kcal/m.h. .degree. C.) Bricks = 2.35 Coating = 1.0 Iron Cladding Temperature Coating Thickness ______________________________________ 100.degree. C. 864 mm 130 576 160 396 190 278 220 195 250 136 280 92 310 58 340 32 370 11 ______________________________________ *From Report T10 of the Fuel Specialist Committee of the Cement Association, P.46 "A Survey Relating to Refractory Materials for Use in Rotary Kilns" March 1972.